1. Technical Field
The present invention relates to a process for correcting a wavefront analyzer. It also relates to a wavefront analyzer in which this process is implemented.
The field of the invention is that of the wavefront analyzers functioning by direct measurement on a beam. The analyzers concerned are of the following types:                Hartmann, comprising an array of holes arranged in front of an array detector, each hole forming, from an incident wavefront, a spot on the detector,        Shack Hartmann or Hartmann Shack, comprising an array of microlenses arranged in front of an array detector, each microlens forming, from an incident wavefront, a spot on the detector, or        Shearing interferometer (or multiple-wave interferometer), comprising means for generating spatially offset replicas of an incident wavefront and means for making the replicas interfere on an array detector in the form of interference spots or fringes.        
2. State of the Prior Art
For all these analyzers, the position of the spots (the Hartmann or Shack Hartmann case) and the position of the interference fringes or of the interference spots (the case of the shearing interferometer) on the detector makes it possible to determine, by an interpretation of the energy signal received on the detector, the local gradients (the local derivatives) of the phase of the incident wavefront, which makes it possible to determine the curvature of the incident wavefront. However, if the shape the spots (or fringes) is disturbed by a local variation of intensity of the analyzed wavefront, the measurement of the local phase gradient is itself distorted.
A local variation of intensity gives rise to an error in the local measurement of the phase gradient of the wavefront.
In fact, in the case of the Hartmann system and with reference to FIG. 1, a variation of intensity at the scale of a micro-hole appears again on the spot measured by the array detector (typically CCD or CMOS) and the position of the energy barycentre of the spot is no longer the exact reflection of the movement of the spot related solely to the modification of the local phase gradient of the analyzed wavefront.
In the case where the incident phase on the micro-hole is flat, the result of the determination of the local phase gradient should give a zero gradient. However, the local variation of the intensity shifts the energy barycentre and the result of the computation gives a non-zero gradient. This difference is the error related to the variation of intensity in front of the micro-hole.
In the case of the Shack-Hartmann technology and with reference to FIGS. 2 and 3, the principle is very similar. However, the presence of microlenses changes the behaviour a little. In fact when the plane of the detector is in the focusing plane of a microlens, the position of the energy barycentre is not dependent on the distribution of intensity in front of the microlens.
In this configuration, the measurement of the phase gradient is completely independent of the intensity gradients. Unfortunately, this property is true only for a single operating position of the detector with respect to the microlenses, and for a given wavelength of the incident wave. The operating position is adjusted, in general, during the setup for collimated beams (zero local curvature in front of the microlens), i.e. the detector is placed in the focal plane of the microlenses. Document [1], referenced on the last page, can be mentioned on this matter.
However, when the beam exhibits aberrations (curvature, astigmatism, coma, spherical aberration, etc.), the local curvature of the wavefront in front of each microlens is no longer zero and the focusing point of the microlens is no longer in the plane of the detector. In this configuration, a variation of intensity in front of the microlens causes a modification of the position of the energy barycentre of the spot.
The error related to the local variation of intensity of the wavefront in front of each microlens therefore depends on the local curvature of the wavefront in front of said microlens. The further the focusing point is from the detector, the greater is the influence of the intensity distribution.
The influence of the variation of intensity on the determination of the local phase gradients of the wavefront in the case of the shearing interferometer (lateral shift interferometer) is very similar to the Hartmann technology case. In fact, the principle of the lateral shift interferometer is to make at least two replicas of the incident wave interfere with each other by shifting them laterally with respect to each other.
Documents [2] and [3] referenced on the last page can be mentioned in particular on this matter.
The interference phenomenon brings the phase term of the incident wave (that which is to be measured) into play and also the amplitude of the wave (related to the intensity). The interference pattern is therefore modified by a variation of intensity of the incident wavefront and the computation of the phase gradient is thereby disturbed.
A principal purpose of the present invention is to overcome these drawbacks by proposing a process for correcting a wavefront analyzer.